The aspects of the disclosed embodiments relate to the field of structures comprising stiffened panels. This relates to panel essentially comprised of a thin coating, whose structural stability is ensured by elements relating to the coating.
More particularly, the disclosed embodiments relate to structures in which the stiffened panels, such as those used for producing the aircraft fuselage, are assembled at the junction of said panels without stiffener continuity.
For explanatory purposes regarding the state of art relating to the assembly of stiffened panels, such as, for example, one embodiment of the disclosed embodiments, the case of the assembly of stiffened panels aimed at producing an aircraft fuselage shall be described.
An aircraft fuselage is typically found in so-called “hull” structures, in particular for reasons related to lighter masses, which are essential in the field of aeronautics. On conventional aircraft, the fuselage comprises one substantially constant section on one part of its length, which gives the fuselage an overall well-known cylindrical shape.
For industrial and logistic reasons, such a fuselage is often produced from multiple cylindrical sections or from multiple section panels according to the type of assembly provided for by the industrial process. When the fuselage is produced from multiple sections, each section can itself be comprised of multiple panels.
In order to produce a rigid but light-weight structure, the hull structures generally comprise a relatively thin coating located on the inner wall of the cylinder. This coating is also referred to as skin. The hull structures also comprise structural elements attached to said coating, aimed at ensuring both the resistance and stability of said structures. In one aircraft fuselage structure, the structural elements substantially directed in the direction of the fuselage cylinder generators are referred to as stiffeners. The structural elements located on a substantially normal plane to said generators are referred to as frames.
With aircraft and for aerodynamic reasons, the stiffeners and frames and generally located inside the fuselage, and are therefore attached to the coating, itself located on the inner wall of the sections of fuselage.
When the panels are assembled to form a section or a fuselage, these panels are generally already equipped with stiffeners. The stiffeners therefore stop at the edges of the panel located on the same side as the stiffener ends, said panel edges being referred to, by extension, as panel ends. These panels comprised of a coating and stiffeners, are referred to as self-stiffened panels.
During the assembly operation for the sections or panels in order to produce a fuselage, the stresses in the coating and stiffeners must be transmitted from one section to the other or from one panel to the other. This refers in particular to tensile, compressive and/or shearing stresses.
In order to ensure the transfer of these stresses, one solution consists in creating coating and stiffener continuity at the level of a panel junction. This continuity is obtained by means of junction parts, on the one hand from panel to panel, and on the other hand from stiffener to stiffener. Said junction parts are produced in such a way as to preserve a transversal cross-section and constant inertia over the entire length of the structure.
One method generally used consists in attaching a plate to the coating of the two panels, said plate taking on the curve of the panels and partially covering the two panels positioned end to end. When the fuselage is assembled in sections, each plate, referred to as shroud, covers all or part of the fuselage perimeter at the level of the junction.
In order to create a connection between two stiffeners located opposite each other at the level of the junction, a specific part, referred to as a batten, is attached between the two stiffeners. A batten has a cross-section generally similar to that of the stiffeners concerned by the junction and covers each stiffener over a long enough distance to efficiently transmit the stresses from one stiffener to the other.
One of the difficulties connected to this type of junction is due to the poor alignment of the stiffeners that must be assembled at the level of a junction.
The unavoidable provisions regarding the dimensions of the stiffeners and their positions on the coatings, connected to the panel manufacturing and assembly methods, do not guarantee the precise alignment of the stiffeners between the two panels or the two sections being assembled.
One known solution consists in not attaching the stiffeners to the coatings over their entire length. A long enough length is left free at the ends of said stiffeners that must be battened. After having positioned the panels to be assembled, the stiffeners can thus be distorted within the limits of their field of elasticity, in order to align them before completing their assembly to the panels and batten.
This solution therefore requires particular assembly operations and cannot be performed in situations where the stiffeners are attached over their entire length, as, for example, with welded or bonded stiffeners, in particular in the case of structures made out of composite materials.
In these cases, the panels must be produced with very strict dimensional tolerances. This solution is limited to large-scale dimensions and always turns out to be very expensive. The alignment faults can also be corrected with blocks. The implementation of said blocks is a delicate and long procedure, requiring, when using a polymerisable mastic, a waiting time detrimental to the duration of the assemblies.
In addition, some stiffener shapes, particularly used in structures made out of composite materials, have closed cross-sections. Once assembled with a panel, the inside of the stiffener can no longer be accessed. This is the general case for stiffeners comprising two sole plates and a body connecting said two sole plates. These stiffeners are referred to as omega-shaped stiffeners due to their characteristic cross-section with a shape similar to a capital omega (Ω).
Without any possibility of inspecting the inside of such stiffeners, attachments are not recommended for unblocking the inside of these stiffeners.
Moreover, in some instances, the junction is produced in the presence of a reinforcing frame. In these cases, in order to ensure the passage of battens from one end of the stiffener to the other, openings must be made through the reinforcing frame. These openings reduce the level of structural resistance of the reinforcing frame. In addition, the presence of the frame significantly increases the complexity of the batten assembly operations and that of the section assembly operations.